Ultrasonic devices may be used in a wide variety of applications, such as acoustic pressure sensors for ultrasonic field characterization. A hydrophone is a type of ultrasonic device that has been employed as an acoustic pressure sensor in the medical field to calibrate an ultrasonic transducer used in medical diagnosis and therapy. Calibration of the ultrasonic transducer can be achieved by directing waves from the transducer to the hydrophone. The hydrophone is operated to provide a quantitative assessment of the characteristics of ultrasonic fields that are created by the transducer in a liquid, such as water.
Performance properties such as sensitivity, frequency response, acoustic transparency and immunity to rf interference must be considered in the design of a hydrophone for ultrasonic field characterization. One type of hydrophone design is a needle-like device described in U.S. Pat. No. 4,789,971 to Powers et. al. Despite the small size of the needle-like hydrophone, this type unavoidably changes the ultrasonic field that is to be characterized. Perturbations of the field are generated as a result of the geometry of the hydrophone and the substantial difference in acoustic impedance between the hydrophone and the liquid in which the hydrophone is immersed.
A membrane hydrophone is a type of device that is generally more acoustically transparent than the needle-like devices. Membrane hydrophones are described in U.S. Pat. Nos. 4,433,400 to DeReggi et. al. and 4,653,036 to Harris et. al. Such hydrophones typically include a thin polyvinylidene fluoride (PVDF) film that is held taut by a rigid hoop. PVDF membranes are employed because the acoustic impedance of PVDF is relatively close to that of water. Impedance matching reduces the reflections generated by the hydrophone. Moreover, the diameter of the hoop is typically several times as large as the diameter of the acoustic beams that are to be encountered, so that the hoop is less likely to generate perturbations.
Shielding from rf interference can be achieved by employing a bilaminate design, rather than a single-sheet PVDF design. The bilaminate structure allows the external surfaces of the membrane to be coated with a metallization which is at ground potential.
Membrane hydrophones having an acceptable sensitivity over a frequency range of 0.5 MHz to 15 MHz are well known. The size of the active sensing area should be smaller than the wavelength of the highest frequency to be encountered. To this end, only a small area of the PVDF membrane is poled using a combination of elevated temperature and a nominal applied electric field. The poling process provides a strongly piezoelectrically active area. The above-cited patent to DeReggi et. al. describes a centrally located active area having a diameter of 0.5 mm.
The thickness of the PVDF membrane also plays a role in determining the frequency response performance of the hydrophone. A thin membrane will have a higher operating frequency than a thick membrane. However, there is a tradeoff between frequency response and mechanical rigidity. A thin membrane is susceptible to breakage since, as previously noted, the hydrophone should have a diameter that is several times as large as the diameter of the ultrasonic field that is to be characterized. A standard thickness of a hydrophone PVDF film is 25 .mu.m.
It is an object of the present invention to provide an acoustic membrane device, such as a membrane hydrophone, having an extended frequency range without a loss of mechanical rigidity.